北京化工大学：《材料导论》课程阅读材料（复合材料）Polymer_nanotechnology-_Nanocomposites.pdf

Feature Article Polymer nanotechnology: Nanocomposites D.R. Paul a,1 , L.M. Robeson b,* aDepartment of Chemical Engineering and Texas Materials Institute, University of Texas at Austin, Austin, TX 78712, United States b Lehigh University, 1801 Mill Creek Road, Macungie, PA 18062, United States article info Article history: Received 19 February 2008 Received in revised form 2 April 2008 Accepted 4 April 2008 Available online xxx Keywords: Nanotechnology Nanocomposites Exfoliated clay abstract In the large field of nanotechnology, polymer matrix based nanocomposites have become a prominent area of current research and development. Exfoliated clay-based nanocomposites have dominated the polymer literature but there are a large number of other significant areas of current and emerging interest. This review will detail the technology involved with exfoliated clay-based nanocomposites and also include other important areas including barrier properties, flammability resistance, biomedical applications, electrical/electronic/optoelectronic applications and fuel cell interests. The important question of the ‘‘nano-effect’’ of nanoparticle or fiber inclusion relative to their larger scale counterparts is addressed relative to crystallization and glass transition behavior. Of course, other polymer (and composite)-based properties derive benefits from nanoscale filler or fiber addition and these are addressed. 2008 Elsevier Ltd. All rights reserved. 1. Introduction The field of nanotechnology is one of the most popular areas for current research and development in basically all technical disciplines. This obviously includes polymer science and technology and even in this field the investigations cover a broad range of topics. This would include microelectronics (which could now be referred to as nanoelectronics) as the critical dimension scale for modern devices is now below 100 nm. Other areas include polymer-based biomaterials, nanoparticle drug delivery, miniemulsion particles, fuel cell electrode polymer bound catalysts, layer-by-layer self-assembled polymer films, electrospun nanofibers, imprint lithography, polymer blends and nanocomposites. Even in the field of nanocomposites, many diverse topics exist including composite reinforcement, barrier properties, flame resistance, electro-optical properties, cosmetic applications, bactericidal properties. Nanotechnology is not new to polymer science as prior studies before the age of nanotechnology involved nanoscale dimensions but were not specifically referred to as nanotechnology until recently. Phase separated polymer blends often achieve nanoscale phase dimensions; block copolymer domain morphology is usually at the nanoscale level; asymmetric membranes often have nanoscale void structure, miniemulsion particles are below 100 nm; and interfacial phenomena in blends and composites involve nanoscale dimensions. Even with nanocomposites, carbon black reinforcement of elastomers, colloidal silica modification and even naturally occurring fiber (e.g., asbestos-nanoscale fiber diameter) reinforcement are subjects that have been investigated for decades. Almost lost in the present nanocomposite discussions are the organic–inorganic nanocomposites based on sol–gel chemistry which have been investigated for several decades [1–3]. In essence, the nanoscale of dimensions is the transition zone between the macrolevel and the molecular level. Recent interest in polymer matrix based nanocomposites has emerged initially with interesting observations involving exfoliated clay and more recent studies with carbon nanotubes, carbon nanofibers, exfoliated graphite (graphene), nanocrystalline metals and a host of additional nanoscale inorganic filler or fiber modifications. This review will discuss polymer matrix based nanocomposites with exfoliated clay being one of the key modifications. While the reinforcement aspects of nanocomposites are the primary area of interest, a number of other properties and potential applications are important including barrier properties, flammability resistance, electrical/electronic properties, membrane properties, polymer blend compatibilization. An important consideration in this review involves the comparison of properties of nanoscale dimensions relative to larger scale dimensions. The synergistic advantage of nanoscale dimensions (‘‘nano-effect’’) relative to larger scale modification is an important consideration. Understanding the property changes as the particle (or fiber) dimensions decrease to the nanoscale level is important to optimize the resultant nanocomposite. As will be noted, many nanocomposite systems noted in the literature can still be modeled using continuum models where absolute size is not important since only shape and volume fraction * Corresponding author. Tel.: þ1 610 481 0117. E-mail addresses: drp@che.utexas.edu (D.R. Paul), lesrob2@verizon.net (L.M. Robeson). 1 Tel.: þ1 512 471 5392. Contents lists available at ScienceDirect Polymer journal homepage: www.elsevier.com/locate/polymer ARTICLE IN PRESS 0032-3861/$ – see front matter 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymer.2008.04.017 Polymer xxx (2008) 1–18 Please cite this article in press as: Paul DR, Robeson LM, Polymer (2008), doi:10.1016/j.polymer.2008.04.017

loading are necessary to predict properties. Nanoscale is considered where the dimensions of the particle, platelet or fiber modification are in the range of 1–100 nm. With the platelet or fiber, the smallest dimension is considered for that range (platelet thickness or fiber diameter). 2. Fundamental considerations In the area of nanotechnology, polymer matrix based nanocomposites have generated a significant amount of attention in the recent literature. This area emerged with the recognition that exfoliated clays could yield significant mechanical property advantages as a modification of polymeric systems [4–6]. The achieved results were at least initially viewed as unexpected (‘‘nano-effect’’) offering improved properties over that expected from continuum mechanics predictions. More recent results have, however, indicated that while the property profile is interesting, the clay-based nanocomposites often obey continuum mechanics predictions. There are situations where nanocomposites can exhibit properties not expected with larger scale particulate reinforcements. It is now well-recognized that the crystallization rate and degree of crystallinity can be influenced by crystallization in confined spaces. In these cases, the dimensions available for spherulitic growth are confined such that primary nuclei are not present for heterogeneous crystallization and homogeneous nucleation thus results. This results in the value of n in the Avrami equation approaching one and often leads to reduced crystallization rate, degree of crystallinity and melting point. This has been observed in phase separated block copolymers [7,8] and has also been observed in polymer blends [9]. Confined crystallization of linear polyethylene in nanoporous alumina showed homogeneous nucleation with pore diameters of 62–110 nm but heterogeneous nucleation for 15–48 nm pores [10]. Linear polyethylene [11] and syndiotactic polystyrene [12] in nanoporous alumina both showed decreased crystallinity versus bulk crystallization. With nanoparticle incorporation in a polymer matrix, similarities to confined crystallinity (as noted above for crystallization in nanopores) exist as well as nucleation effects and disruption of attainable spherulite size. With inorganic particle and nanoparticle inclusions, nucleation of crystallization can occur. At the nanodimension scale, the nanoparticle can substitute for the absence of primary nuclei thus competing with the confined crystallization. At higher nanoparticle content, the increased viscosity (decreased chain diffusion rate) can lead to decreased crystallization kinetics. Thus, the crystallization process is complex and influenced by several competing factors. Nucleation of crystallization (at low levels of addition) evidenced by the onset temperature of crystallization (Tc) and crystallization half-time has been observed in various nanocomposites (poly- (3-caprolactone)–nanoclay [13], polyamide 66–nanoclay [14,15], polylactide–nanoclay [16], polyamide 6–nanoclay [17], polyamide 66–multi-walled carbon nanotube [18], polyester–nanoclay [19], poly(butylene terephthalate)–nanoclay [20], polypropylene–nanoclay (sepiolite) [21], polypropylene/multi-walled carbon nanotube [22]). At higher levels of nanoparticle addition, retardation of the crystallization rate has been observed even in those systems where nucleation was observed at low levels of nanoparticle incorporation [15,18,20,22–24]. The higher level of nanoparticle inclusion was noted to yield retardation of crystallization due to diffusion constraints. This was also apparent in a study where unmodified and organically modified clay were incorporated in maleic anhydride grafted polypropylene [25]. Nucleation was observed with unmodified clay, whereas the exfoliated clay yielded a reduced crystallization rate. A recent review of the crystallization behavior of layered silicate clay nanocomposites noted that while nucleation is observed in many systems the overall crystallization rate is generally reduced particularly at higher levels of nanoclay addition [26]. Another ‘‘nano-effect’’ noted in the literature has been the change in the Tg of the polymer matrix with the addition of nanosized particles. Both increases and decreases in the Tg have been reported dependant upon the interaction between the matrix and the particle. In essence, if the addition of a particle to an amorphous polymer leads to a change in the Tg, the resultant effect on the composite properties would be considered a ‘‘nano-effect’’ and not predictable employing continuum mechanics relationships unless the Tg changes were properly accounted for or were quite minor. The glass transition of a polymer will be affected by its environment when the chain is within several nanometers of another phase. An extreme case of this is where the other environment is air (or vacuum). It has been well-recognized in the literature that the Tg of a polymer at the air–polymer surface or thin films (<100 nm) may be lower than that in bulk [27]. This can also be considered a con- finement effect. A specific experimental example was reported where poly(2-vinyl pyridine) showed an increase in Tg, poly(methyl methacrylate) (PMMA) showed a decrease in Tg and polystyrene showed no change with silica nanosphere incorporation. These differences were ascribed to surface wetting [28]. The Tg decrease for PMMA was ascribed to free volume existing at the polymer surface interface due to poor wetting. In most literature examples where Tg values have been obtained, usually only modest changes are reported (<10 C) as noted in various examples tabulated in Table 1. In some cases the organic modification of clay can result in a decrease in Tg due to plasticization [29]. It should be noted that the values noted in Table 1 involved relatively low levels of nanoparticle incorporation (<0.10 wt fraction and even lower volume fraction) and larger changes in Tg could be expected at much higher volume fraction loadings. For crosslinked polymers, another consideration is necessary as the presence of nanoparticles could yield a crosslink density change over the unmodified composite. This could be due to preferential interactions of the crosslinking agent with the nanoparticle surface or interruption of the crosslink density due to confinement effects. A theoretical model has been Table 1 Glass transition changes with nanofiller incorporation Polymer Nanofiller Tg change (C) Reference Polystyrene SWCNT 3 [34] Polycarbonate SiC (0.5–1.5 wt%) (20–60 nm particles) No change [35] Poly(vinyl chloride) Exfoliated clay (MMT) (<10 wt%) 1 to 3 [36] Poly(dimethyl siloxane) Silica (2–3 nm) 10 [37] Poly(propylene carbonate) Nanoclay (4 wt%) 13 [38] Poly(methyl methacrylate) Nanoclay (2.5–15.1 wt%) 4–13 [39] Polyimide MWCNT (0.25–6.98 wt%) 4 to 8 [40] Polystyrene Nanoclay (5 wt%) 6.7 [41] Natural rubber Nanoclay (5 wt%) 3 [42] Poly(butylene terephthalate) Mica (3 wt%) 6 [43] Polylactide Nanoclay (3 wt%) 1 to 4 [29] SWCNT ¼single-walled carbon nanotubes; MMT ¼ montmorillonite; MWCNT ¼ multi-walled carbon nanotubes. 2 D.R. Paul, L.M. Robeson / Polymer xxx (2008) 1–18 ARTICLE IN PRESS Please cite this article in press as: Paul DR, Robeson LM, Polymer (2008), doi:10.1016/j.polymer.2008.04.017

developed to predict the glass transition temperature of nanocomposites [30]. The model predicts both increases and decreases in Tg dependant upon specific interactions and shows good agreement with the experimental data noted above [28]. A situation does exist where significant increases in the glass transition temperature have been noted involving polyhedral oligomeric silsesquioxane (POSS) cage structures chemically reacted into the polymeric network [31–34]. These cage structures with a particle diameter in the range of 1–3 nm can be functionalized to provide chemical reactivity with various polymer systems. Examples include octavinyl (R ¼ vinyl group) incorporation for copolymerization with PMMA [31], amine groups for incorporation into polyamides [32] and polyimides [33]. This parallels the glass transition increase often noted in the sol–gel inorganic–organic networks. While the glass transition temperature and crystallinity are the major property changes of interest of the nanocomposite polymer matrix, other ‘‘nano-effects’’ or property improvements over larger scale dimensions can be observed. Disruption of packing of rigid chain polymers resulting in higher free volume has been observed in permeability studies [44], surface area effects in photovoltaic applications involving conjugated polymers, surface area effects for catalysts incorporated in polymers, polymer chain dimensions where the radius of gyration is greater than the distance between adjacent nanoparticles, optical properties, nanofiber scaffolds for tissue engineering are additional areas. The ‘‘aging’’ of polymers is a thickness dependant property with rapid change at nanoscale dimensions [45,46]. This property is due to the ability of free volume to diffuse out of the sample and the diffusion coefficient (although very low) becomes important in the time scale associated with polymer utility (days to years) at nanoscale thicknesses. Surface area effects including catalysts, bioactivity, often require nanolevel dimensions to achieve optimum performance. This review of polymer matrix based nanocomposites is divided into two major sections: clay-based nanocomposites with emphasis on mechanical reinforcement and other property modifications. Mechanical enhancement is usually associated with polymer-based composites, however, a number of other areas have emerged where additional property enhancements can be realized by incorporation of nanoscale particles, platelets or fibers. 3. Clay-based polymer nanocomposites 3.1. Structure of montmorillonite The clay known as montmorillonite consists of platelets with an inner octahedral layer sandwiched between two silicate tetrahedral layers [47] as illustrated in Fig. 1. The octahedral layer may be thought of as an aluminum oxide sheet where some of the aluminum atoms have been replaced with magnesium; the difference in valences of Al and Mg creates negative charges distributed within the plane of the platelets that are balanced by positive counterions, typically sodium ions, located between the platelets or in the galleries as shown in Fig. 1. In its natural state, this clay exists as stacks of many platelets. Hydration of the sodium ions causes the galleries to expand and the clay to swell; indeed, these platelets can be fully dispersed in water. The sodium ions can be exchanged with organic cations, such as those from an ammonium salt, to form an organoclay [48–57]. The ammonium cation may have hydrocarbon tails and other groups attached and is referred to as a ‘‘surfactant’’ owing to its amphiphilic nature. The extent of the negative charge of the clay is characterized by the cation exchange capacity, i.e., CEC. The X-ray d-spacing of completely dry sodium montmorillonite is 0.96 nm while the platelet itself is about 0.94 nm thick [47,58]. When the sodium is replaced with much larger organic surfactants, the gallery expands and the X-ray d-spacing may increase by as POSS Si O Si O Si O Si O Si O O O O Si O O Si O Si O R R R R R R R R R = alkyl, aryl, cycloaliphatic, vinyl, amino, nitrile halogen. alcohol, ester, isocyanate, glycidyl etc. Tetrahedral sheet Na+ 2:1 Layer Octahedral sheet Tetrahedral sheet Interlayer or gallery Fig. 1. Structure of sodium montmorillonite. Courtesy of Southern Clay Products, Inc. D.R. Paul, L.M. Robeson / Polymer xxx (2008) 1–18 3 ARTICLE IN PRESS Please cite this article in press as: Paul DR, Robeson LM, Polymer (2008), doi:10.1016/j.polymer.2008.04.017

much as 2 to 3-fold [59,60]. While the thickness of montmorillonite platelets is a well-defined crystallographic dimension, the lateral dimensions of the platelets are not. They depend on how the platelets grew from solution in the geological process that formed them. Many authors grossly exaggerate the lateral size with dimensions quoted of the order of microns or even tens of microns. A commonly used montmorillonite was accurately characterized recently by depositing platelets on a mica surface from a very dilute suspension and then measuring the lateral dimensions by atomic force microscopy [61]. Since the platelets are not uniform or regular in lateral size or shape, the platelet area, A, was measured and its square-root was normalized by platelet thickness, t, to calculate an ‘‘aspect ratio’’. The distribution of aspect ratios found is shown in Fig. 2. If each platelet were circular with diameter D, then ffiffiffi A p =t would be ffiffiffiffiffiffiffiffiffi p=4 p ðD=tÞ ¼ 0:89ðD=tÞ. Since t is approximately 1 nm, Fig. 2 shows that the most probable lateral dimension is in the range of 100–200 nm. 3.2. Nanocomposite formation: exfoliation Nanocomposites can, in principle, be formed from clays and organoclays in a number of ways including various in situ polymerization [4,6,62–68], solution [51,53], and latex [69,70] methods. However, the greatest interest has involved melt processing [71– 139] because this is generally considered more economical, more flexible for formulation, and involves compounding and fabrication facilities commonly used in commercial practice. For most purposes, complete exfoliation of the clay platelets, i.e., separation of platelets from one another and dispersed individually in the polymer matrix, is the desired goal of the formation process. However, this ideal morphology is frequently not achieved and varying degrees of dispersion are more common. While far from a completely accurate or descriptive nomenclature, the literature commonly refers to three types of morphology: immiscible (conventional or microcomposite), intercalated, and miscible or exfoliated. These are illustrated schematically in Fig. 3 along with example transmission electron microscopic, TEM, images and the expected wide angle X-ray scans [48–53,83]. For the case called ‘‘immiscible’’ in Fig. 3, the organoclay platelets exist in particles comprised of tactoids or aggregates of tactoids more or less as they were in the organoclay powder, i.e., no separation of platelets. Thus, the wide angle X-ray scan of the polymer composite is expected to look essentially the same as that obtained for the organoclay powder; there is no shifting of the Xray d-spacing. Generally, such scans are made over a low range of angles, 2q, such that any peaks from a crystalline polymer matrix are not seen since they occur at higher angles. For completely exfoliated organoclay, no wide angle X-ray peak is expected for the nanocomposite since there is no regular spacing of the platelets and the distances between platelets would, in any case, be larger than what wide angle X-ray scattering can detect. Often X-ray scans of polymer nanocomposites show a peak reminiscent of the organoclay peak but shifted to lower 2q or larger d-spacing. The fact that there is a peak indicates that the platelets are not exfoliated. The peak shift indicates that the gallery has expanded, and it is usually assumed that polymer chains have entered or have been intercalated in the gallery. Placing polymer chains in such a confined space would involve a significant entropy penalty that presumably must be driven by an energetic attraction between the polymer and the organoclay [76–79]. It is possible that the gallery expansion may in some cases be caused by intercalation of oligomers or low molecular weight polymer chains. The early literature seemed to suggest that ‘‘intercalation’’ would be useful and perhaps a precursor to exfoliation. Subsequent research has suggested alternative ideas about how the exfoliation process may occur in melt processing and how the details of the mixing equipment and conditions alter the state of dispersion achieved [54,82,84,140]. These ideas are summarized in the cartoon shown in Fig. 4 [84]. As made commercially, the particles of an organoclay powder are about 8 mm in size and consist of aggregates of tactoids, or stacks of platelets; the stresses imposed during melt mixing break up aggregates and can shear the stack into smaller ones as suggested in Fig. 4. However, there evidently is a limit to how finely the clay can be dispersed just by mechanical forces. If the polymers and organoclay have an ‘‘affinity’’ for one another, the contact between polymers and organoclay can be increased by peeling the platelets from these stacks one by one until, given enough time in the mixing device, all the platelets are individually dispersed as suggested in Fig. 4. This notion is supported by many TEM images at various locations in the extruder and is more plausible than imagining the polymer chains diffusing into the galleries, i.e., intercalation, and eventually pushing them further and further apart until an exfoliated state is reached. The nature of the extruder and the screw configuration are important to achieve good organoclay dispersion [83]. Longer residence times in the extruder favor better dispersion [83]. In some cases, having a higher melt viscosity is helpful in achieving dispersion apparently because of the higher stresses that can be imposed on the clay particles [84,126]; however, this effect is not universally observed. The location of where the organoclay is introduced into the extruder has also been shown to be important [120]. However, no matter how well these process considerations are optimized, it is clear that complete exfoliation, or nearly so, cannot be achieved unless there is a good thermodynamic affinity between the organoclay and the polymer matrix. This affinity can be affected to a very significant extent by optimizing the structure of the surfactant used to form the organoclay [85,88,99,100,109,113,119,141] and possibly certain features of the clay itself like its CEC [115], as this affects the density of surfactant molecules over the silicate surface. A key factor in the polymer–organoclay interaction is the af- finity polymer segments have for the silicate surface [84,85,94,113,141,142]. Nylon 6 appears to have good affinity for the silicate surface, perhaps by hydrogen bonding, and as a result very high levels of exfoliation can be achieved in this matrix provided the processing conditions and melt rheology are properly selected [83,84,120]. Surfactants with a single long alkyl tail give the best exfoliation [141]. As more long chain alkyls are added to the surfactant, the extent of exfoliation is decreased [141]. It has been proposed that at least one alkyl tail is needed to reduce the platelet–platelet cohesion while adding more than one tends to block Fig. 2. Aspect ratio distribution of native sodium montmorillonite platelets [61]. Reproduced with permission of the American Chemical Society. 4 D.R. Paul, L.M. Robeson / Polymer xxx (2008) 1–18 ARTICLE IN PRESS Please cite this article in press as: Paul DR, Robeson LM, Polymer (2008), doi:10.1016/j.polymer.2008.04.017